refactor actor_user function to use imported deep_get

global_helpers/panther_azuresignin_helpers.py

@@ -1,9 +1,14 @@
+from panther_base_helpers import deep_get
+
+
 def actor_user(event):
+    # 'event' could be a PantherEvent or an ImmutableCaseInsensitiveDict, so we have to use the
+    #   imported deep_get method.
     category = event.get("category", "")
     if category in {"ServicePrincipalSignInLogs"}:
-        return event.deep_get("properties", "servicePrincipalName")
+        return deep_get(event, "properties", "servicePrincipalName")
     if category in {"SignInLogs", "NonInteractiveUserSignInLogs"}:
-        return event.deep_get("properties", "userPrincipalName")
+        return deep_get(event, "properties", "userPrincipalName")
     return None